1. Field of the Invention
The invention relates to an optical communication system, specifically an optical communication system comprising a cladding pumped fiber laser.
2. Discussion of the Related Art
Recently, optical power from diode-laser arrays has been improved by using the arrays to pump a cladding pumped fiber, and in turn employing the output of the pumped fiber with the concomitant improvement. See, e.g., S. Grubb et al., "Fiber Raman lasers emit at many wavelengths," Laser Focus World, February 1996, at 127 and L. Zentano, "High-Power Double-Clad Fiber Lasers," Journal of Lightwave Technology, Vol. 11, No. 9, September 1993, the disclosures of which are herein incorporated by reference. Cladding pumped fiber lasers (i.e., the combination of a light source and a cladding pumped fiber) are advantageous in that they allow the coupling and magnification of light from high-power diode-laser arrays into a single mode fiber. Specifically, as reflected in FIG. 1, cladding pumped fiber lasers rely on a relatively large, separately light-guiding pump cladding 14 (i.e., outer waveguide) in the cladding pumped fiber 10, the cladding 14 surrounding a smaller doped (typically with a rare earth element) single-mode core 12. A light source, e.g., a diode-laser array, directs pump light into the pump cladding 14, and, advantageously, the primary pump-light loss mechanism is absorption of light into the core 12, where the amplification and lasing occurs. The core 12 has a much smaller cross-sectional area than the pump cladding 14, and, for a certain wattage of pump light, the energy per unit area in the core will thus be higher than in the pump cladding. Absorption of the pump light by the core thereby results in an increase in the brightness. The actual increase in brightness depends on the ratio of pump cladding area to core area--the higher the ratio, the greater the brightness increase. Through use of feedback elements such as dielectric coatings or Bragg gratings, laser power can be extracted from the single mode core and directed into an attached single mode optical fiber that transmits the lased light.
In a typical cladding pumped fiber 10, as shown (not to scale) in FIGS. 1 and 3, the pump cladding 14 is rectangular, having dimensions of about 100 by 360 .mu.m. (In FIG. 3, the longer axis of the pump cladding 14 is oriented vertically.) Other pump cladding configurations are possible, such as that of now abandoned U.S. patent application Ser. No. 08/561,682, filed Nov. 22, 1995 (our Docket No. DiGiovanni 23), the disclosure of which is hereby incorporated by reference. The rare earth-doped core 12 is circular and has a diameter of about 5-10 .mu.m. The pump cladding 14 is typically surrounded either with a single polymer layer, the layer acting both as a cladding and a protective coating, or with two layers, a cladding layer and a protective coating layer. FIG. 1 illustrates the latter. A polymer cladding 16 having a thickness of about 2 .mu.m to about 30 .mu.m, and a protective coating 18 having a thickness of about 30 .mu.m to about 60 .mu.m, surround the cladding 14. The pump cladding 14 is typically formed from silica, and the core 12 is typically formed from silica doped with rare earths such as neodymium, ytterbium, or erbium/ytterbium. The polymer cladding 16 is typically formed from a polymer having a relatively low index of refraction, e.g., a fluorinated polymer having an index of refraction of about 1.38. Typical protective coatings 18 include polymers such as acrylates, which have thermal stabilities around 100.degree. C. (Thermal stability, as used herein, indicates the temperature above which a material experiences irreversible damage or structural change, as measured by the onset of weight loss in the material.)
In a typical single mode transmission fiber 20, shown (not to scale) in FIGS. 2 and 3, the core 22 is circular and has a diameter of about 5-10 .mu.m, and is surrounded by a cladding 24 having a diameter of about 125 .mu.m. The cladding 24 is surrounded by a protective coating 26, e.g., an acrylate, having a thickness of about 50 to about 70 .mu.m. The core 22 of such a typical single mode fiber is doped silica, the dopant producing a relatively high index of refraction, typically about 1.467, and the cladding 24 is silica having a lower index of refraction, typically about 1.465. FIG. 3 shows a cross-sectional side view reflecting the approximate relationship when cladding pumped fiber 10 and single mode fiber 20 are spliced together. The Figure indicates the areas at which the pump cladding 14 tends to overlap the cladding 24 and protective coating 26 of the single mode fiber 20.
While good results have been obtained from cladding pumped fiber lasers, improvements in the overall systems containing such lasers are desired and sought.